The present invention relates generally to improvements in digital wireless telephony. More particularly, the invention relates to advantageous aspects of receivers and transmitters operative to detect and correct errors found in Baudot-encoded text telephony signals in a digital wireless telephony system.
Text telephony (TTY) is a technique developed to aid communication by persons who have difficulty using ordinary telephone equipment, typically because of difficulties with speech or hearing. Communication is accomplished by transmitting text messages over the telephone network, using special equipment to allow generation, transmission and display of text. The typical text telephony connection allows the choice of two-way text communication, or the use of voice communication by one party and text communication by the other.
TTY requires special terminal devices on both ends of the telephone line. One possible device is a teletype, which consists of a keyboard and a display, sometimes also including a printer. The teletype is connected to a telephone set via an acoustic coupler or an RJ11 connector. Some teletypes are portable and lightweight, sometimes weighing less than a pound, and are well suited to work with mobile terminals. Another suitable device is an integrated teletype and telephone, which can perform both voice and text telephony. An additional existing device is a TTY modem which converts a personal computer (PC) into a TTY device. This modem serves as an interface between the PC and the telephone line, and allows the PC to operate as a TTY device. In particular, it performs encoding and decoding between ASCII character streams and TTY signals. A further device is an analog cellular phone adapter. This device is a brand and model specific adapter which connects the RJ11 port on a teletype to a connection in a digital cellular phone.
In text telephony, the text messages are usually encoded using frequency shift key (FSK) modulation, and are transmitted over the telephone line and decoded at a receiver. TTY signals are transparent to the telephone network and are treated like any other audio signal.
Several coding schemes are available for use with TTY. The most common is Baudot coding, which is supported as a default by all or almost all TTY terminals.
TTY techniques are well developed in the context of conventional wire telephony. Wireless telephony, however, presents at least two significant issues which must be addressed if TTY is to be successful. One issue is the distortion to the TTY signals caused by the wireless channel, and a second issue is the need to provide an interface between the cellular phone and a TTY terminal. These issues have been satisfactorily dealt with in the case of analog phones. A number of manufacturers offer TTY interface gear for use between the cellular phone handset and an RJ11 port of a teletype unit. In addition, distortion caused by analog wireless communication is generally acceptable for transmission of Baudot coded messages.
Digital wireless communication provides numerous advantages for voice communication. However, prior-art digital wireless communication has not proven to be well suited for TTY communication, because the FSK coding is very sensitive to the channel distortion in digital wireless systems, to the point that practical TTY communication is prevented. There are two causes for this problem. In commonly used speech coders used in systems following the IS-136 standard (which employs time division multiple access, or TDMA), source coding distortion is introduced into the Baudot coding process. The speech coders in question are IS-641 and VSELP. The coders used in IS-136 have a bit rate of 8 kb/sec, in contrast to the US-1 coder used in GSM-NA (PCS1900) or the QCELP13 coder of IS-95 (which employs code division multiple access, or CDMA), which provide bit rates of 12-13 kb/sec, and produce significantly less source coding distortion. The IS-136 coder is sufficiently widely used that it will continue to be encountered as part of the communications infrastructure, and therefore any TTY communications system must be able to deal with source code distortions produced by IS-136 systems. On the other hand, the channel error rate in the IS-95 protocol usually precludes useful TTY communication.
Improved communication is possible if methods of error detection and correction can be introduced into communication using Baudot coding. This may be advantageously done by adding control information to message information. Commonly used error correction protocols used in other contexts include, for example, the use of cyclic redundancy code (CRC) information which is added to message data by a transmitter and then used by a receiver to check the correctness of the received message.
A large base of equipment presently exists for the use of Baudot coding to transmit TTY messages. The need for compatibility with this equipment base presents a significant obstacle for the use of error correcting codes. Wireless telephones are typically used to communicate with many different telephones without regard to type. A digital wireless telephone which employed Baudot coding would be expected to communicate with any number of other devices, whether these devices were digital wireless, analog wireless, or wireline devices. In error correction systems of the prior art, error correction systems typically require coordination between the transmitter and the receiver. If a transmitter includes error correction code in the transmitted message, the receiver must expect this code in the received message. If error correction code is present in the transmitted message, but is not expected by the receiver, this code will likely be interpreted as data and produce corrupted results. Similarly, if the receiver expects to find error correction code in the received message, but the transmitter does not supply this code, data bits in the received message will be incorrectly interpreted as error correction code and will produce faulty results.
Wireline and analog wireless TTY devices are much less susceptible to errors and distortion than are digital wireless devices. This robustness of operation suggests that error correction systems are likely to remain a low priority to manufacturers of wireline and analog wireless devices, and there will continue to be a large installed base of TTY devices which do not include error correction systems.
Because of the large base of installed equipment, and the likelihood that new equipment which lacks error correction features will continue to be installed, it is important that any TTY device which includes error correction should be able to communicate successfully with equipment which lacks error correction.
The Baudot code is a half duplex code using characters made up of bits represented by designated frequencies, each bit being 22.0xc2x10.4 msec long. A xe2x80x9c1xe2x80x9d (MARK) is represented by a frequency of 1400 Hz while a xe2x80x9c0xe2x80x9d (SPACE) is represented by a frequency of 1800 Hz. A transmitted character includes a start bit (0), five data bits, and one stop bit (1). The duration of the stop bit can be anywhere between a normal bit duration to two normal bits duration. If no other character is transmitted immediately after the current character, then the stop bit may be extended by a xe2x80x9cMARK hold timexe2x80x9d which may be anywhere from 0 to 300 msec.
As noted above, the stop bit in a Baudot character may be of any length from 1 to 2 bits. This gives the stop bit a time duration which may vary anywhere from 22 to 44 msec. This variation in duration provides an opportunity for insertion of non-data information, such as control information.
In Baudot communication, the data rate is at most 32 bits per second. The Baudot code rate is 45 bits per second including a start bit and a stop bit, corresponding to about 6 characters per second. In reality, the rate at which data is transmitted is much lower due to human typing speed limitations. Therefore, even with faster code and more bits per character, a data rate of more than 50 bits per second will not be expected.
In all wireless standards, a full rate speech packet provides at least 100 error protected bits (class 1a and 1b) of which at least 50 are also protected by CRC. This represents a data capacity significantly greater than required to transmit Baudot signals. Some of this excess capacity is therefore available to be adapted for use in error correction. Bits can be xe2x80x9cstolenxe2x80x9d from data transmission in order to serve as error correction codes, without a meaningful reduction in data transmission rates. These xe2x80x9cstolenxe2x80x9d bits, if appropriately handled, may advantageously be designated so that they will not be misinterpreted as data bits by receivers not having error correction capability, but will instead be ignored or skipped over.
There exists, therefore, a need in the art for methods and apparatus for error correction in Baudot code communication equipment, which employs error correction which is transparent to equipment with no error correction capability, and which employs data bits in excess of those needed to convey data at a maximum practical rate, in order to convey information needed for error correction.
In one aspect, a Baudot capable communication system according to the present invention holds Baudot characters in a buffer until either a predetermined number of characters enters the buffer or a predetermined time elapses. Once one of these conditions occurs, the characters are passed to a character packer. The character packer generates a message identifying the buffer contents as a Baudot message and specifying the characters comprising the message. The message will be assigned a CRC with each bit toggled so that the message appears as a bad frame. The duration of the message will be selected within a silence segment or within a stop bit of length 2, so that the loss of a frame will have little impact. After the message is created, the Baudot encoder will then generate the waveforms of the Baudot characters and feed them to a speech encoder for transmission.
If the message is received at a receiver which is not capable of interpreting the error correcting data, the message will appear as encoded frames of silence followed by a single bad frame, followed by the Baudot signal, which will be regenerated as would any other Baudot signal. The bad frame would appear in the midst of silence and would not affect the received message. If, however, the message is received at a receiver having error correction capability according to the present invention, the bad frame will be properly interpreted and the error correcting data in the frame will be read and properly employed.
In another aspect, error identification and correction information is embedded in stop bits of Baudot characters. A stop bit may be adapted to include odd or even parity information, or may alternatively include error correction information. A transmitter according to the present invention may add a stop bit having error correction information to each character transmitted. If a character is received at a conventional receiver, the stop bit will appear and will be interpreted as an ordinary stop bit. However, if a character is received at a receiver according to the present invention, the receiver will correctly interpret error correction information contained in the stop bit.